This invention relates generally to optical disks for recording and reproducing various information signals, such as a video signal, thereon and therefrom by means of an optical disk recording/reproducing apparatus.
Various types of optical disk recording/reproducing apparatus have been known hitherto, and in one type thereof an optical disk, coated with a recording material by spraying or vapor deposition, is rotated, and a tiny spot of laser light beam, which is focussed so that the diameter is less than 1 micrometer, is applied on the optical disk. The intensity of the laser light to be emitted from a laser light source is modulated by a recording signal so that recording of a video signal or a digital signal can be effected in real-time operation as an optical characteristic change such as variation in refractive index, for instance, phase variation by concave-convexo portions on the optical disk, or variation in reflection factor or in transmittance by light and shade variation or formation of holes. The optical characteristic change on the disk will be detected to reproduce prerecorded information.
In such apparatus, structure of an optical disk has been known where recording tracks are formed on an optical disk in advance and then a recording material is formed thereon by spraying or vapor deposition, as a method for actualizing high density of recording tracks, partial writing or erasing at random places, and for removing influence due to vibrations of the disk. Recording or writing is effected by recording information on the groove-like guide tracks, which have been dug or grooved on the surface of an optical disk, in such a manner that tracking control is effected so as to follow the zigzag motion caused by the eccentricity of the guide tracks with the above-mentioned tiny spot focussed on the groove-like track. Since the track density of the guide tracks is defined by the pitch of the guide tracks to be formed irrespective of the eccentricity of the optical disk, a disk of an extremely high density can be manufactured. For instance, a disk having a diameter of 30 centimeters can contain 20,000 to 40,000 coaxial tracks. Since such a large capacity optical disk is capable of containing 20,000 to 40,000 frames of TV (television) pictures, address information is necessary to selectively pickup necessary information. The address information is assigned to all the tracks as serial numbers from the inner-most track to the outer-most track or vice versa.
The address data, of respective tracks are aligned radially where the address datum of each track is formed in the form of an arc subtending a given angle with respect to the center of the disk. Therefore, it is necessary to prevent other information from being recorded on the region carrying the address data. Furthermore, it is also necessary to control the rotation of the optical disk by matching the phase of the vertical synchronous signal of a TV picture signal with the phase of the address information region, so that a TV picture signal is recorded after the end of the address information region in the case of recording a single frame of a TV picture, for example. To this end, an optical disk requires a mark indicative of the position of the starting point of rotation. Although the address information region may be used as such a mark, it is not practical to do so because a detection signal, obtained as a result of searching a track by moving the tiny spot of light at high speed, suffers from irregularity due to noises.
On the other hand, since digital information does not have a predetermined length as does a TV picture, a problem would occur if only address information is provided. Therefore, each track is divided into a plurality of information regions for controlling the digital information. This information region is called a sector, and respective sectors are numbered so that serial numbers are assigned to consecutive sectors in each track. With this arrangement, each sector in the optical disk can be readily designated by its track address and sector address.
It is advantageous to form each of the track address information and the sector address information as a groove-like portion on the optical disk together with the guide tracks in view of manufacturing technique inasmuch as optical disks having guide tracks can be manufactured by stamping in the same process for manufacturing VLP (trademark) disks.
The above-mentioned index mark has been provided hitherto by adhering a piece of aluminum film or vapor-depositing a mirror surface. Therefore, the positional accuracy of the index mark on an optical disk is not very high, while productivity of optical disks is low.